Gamma Cephei
Encyclopedia
Gamma Cephei traditionally named Errai, Er Rai, and or Alrai, is a binary star system
approximately 45 light-year
s away in the constellation
of Cepheus
. Gamma Cephei contains an apparent magnitude
of 3.22. The visible part of the system is a stellar class K1III-IV orange subgiant star on its first ascent off the main sequence. It is currently thought to be 6.6 billion years old (based on Fe/H metallicity).
Gamma Cephei is the naked-eye star that will succeed Polaris
as the Earth's northern pole star
, due to the precession of the equinoxes
. It will be closer to the northern celestial pole
than Polaris around 3000 CE and will make its closest approach around 4000 CE. The "title" will pass to ι Cephei
some time around 5200 CE.
The star has a companion star with a mass approximately 0.409 times that of our Sun. Gamma Cephei B is of stellar mass and is assumed to be of similar age to its primary. It is probably a red dwarf
of class M4, 6.2 degrees of magnitude fainter than the K-type primary star.
الراعي ar-rā‘ī, meaning "the shepherd". Confusingly, the star β Ophiuchi
is sometimes also called Alrai, but it is more commonly known as Cebalrai or Kelb Alrai, meaning "shepherd's dog".
In Chinese
, , meaning Left Wall of Purple Forbidden Enclosure
, refers to an asterism consisting of γ Cephei, ι Draconis
, θ Draconis
, η Draconis
, ζ Draconis
, υ Draconis, 73 Draconis and 23 Cassiopeiae. Consequently, γ Cephei itself is known as , representing , meaning The Second Imperial Guard or Minor Guard
technique later used successfully by others. However, the claim was retracted in 1992 because the quality of the data was not good enough to establish discovery. But in 2002, evidence of the planet was considerably strengthened by new measurements by Artie Hatzes and his collaborators at the McDonald Observatory
.
The secondary star B orbits A at only 9.5 times the semimajor axis of A's planet. This is the smallest ratio of planetary orbits in multiple systems. The orbits are probably coplanar.
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
approximately 45 light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s away in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
of Cepheus
Cepheus (constellation)
Cepheus is a constellation in the northern sky. It is named after Cepheus, King of Aethiopia in Greek mythology. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations.-Stars:...
. Gamma Cephei contains an apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
of 3.22. The visible part of the system is a stellar class K1III-IV orange subgiant star on its first ascent off the main sequence. It is currently thought to be 6.6 billion years old (based on Fe/H metallicity).
Gamma Cephei is the naked-eye star that will succeed Polaris
Polaris
Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....
as the Earth's northern pole star
Pole star
The term "Pole Star" usually refers to Polaris, which is the current northern pole star, also known as the North Star.In general, however, a pole star is a visible star, especially a prominent one, that is approximately aligned with the Earth's axis of rotation; that is, a star whose apparent...
, due to the precession of the equinoxes
Precession of the equinoxes
In astronomy, axial precession is a gravity-induced, slow and continuous change in the orientation of an astronomical body's rotational axis. In particular, it refers to the gradual shift in the orientation of Earth's axis of rotation, which, like a wobbling top, traces out a pair of cones joined...
. It will be closer to the northern celestial pole
Celestial pole
The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, indefinitely extended, intersects the imaginary rotating sphere of stars called the celestial sphere...
than Polaris around 3000 CE and will make its closest approach around 4000 CE. The "title" will pass to ι Cephei
Iota Cephei
Iota Cephei is a Class K0, third-magnitude star in the constellation Cepheus. Iota Cephei is an orange giant star located about 115 light-years from Earth. Due to precession of the equinoxes, it will become the pole star some time around 5200 CE.-External links:...
some time around 5200 CE.
The star has a companion star with a mass approximately 0.409 times that of our Sun. Gamma Cephei B is of stellar mass and is assumed to be of similar age to its primary. It is probably a red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....
of class M4, 6.2 degrees of magnitude fainter than the K-type primary star.
Etymology
Gamma Cephei's traditional name derives from the ArabicArabic language
Arabic is a name applied to the descendants of the Classical Arabic language of the 6th century AD, used most prominently in the Quran, the Islamic Holy Book...
الراعي ar-rā‘ī, meaning "the shepherd". Confusingly, the star β Ophiuchi
Beta Ophiuchi
Beta Ophiuchi is a K-type giant star in the constellation Ophiuchus. It has the traditional names Cebalrai , Cheleb, or Kelb Alrai, or sometimes just Alrai...
is sometimes also called Alrai, but it is more commonly known as Cebalrai or Kelb Alrai, meaning "shepherd's dog".
In Chinese
Chinese language
The Chinese language is a language or language family consisting of varieties which are mutually intelligible to varying degrees. Originally the indigenous languages spoken by the Han Chinese in China, it forms one of the branches of Sino-Tibetan family of languages...
, , meaning Left Wall of Purple Forbidden Enclosure
Purple Forbidden enclosure
The Purple Forbidden enclosure is one of the San Yuan or Three enclosures. Stars and constellations of this group lie near the north celestial pole and visible all year from temperate latitudes in the Northern Hemisphere.-Asterisms:The asterisms are :...
, refers to an asterism consisting of γ Cephei, ι Draconis
Iota Draconis
Iota Draconis is an orange giant star located away in the constellation Draco. A visually unremarkable star of apparent magnitude 3.31, it has recently been discovered to have a planet.-Name:...
, θ Draconis
Theta Draconis
Theta Draconis is a fourth-magnitude star in the constellation Draco.-Chinese name:In Chinese, , meaning Left Wall of Purple Forbidden Enclosure, refers to an asterism consisting of θ Draconis, ι Draconis, η Draconis, ζ Draconis, υ Draconis, 73 Draconis, γ Cephei and 23 Cassiopeiae...
, η Draconis
Eta Draconis
Eta Draconis is a star in the constellation Draco. Eta Draconis has apparent magnitude +2.73 and is a yellow giant variable star belonging to spectral class G8III. The luminosity of Eta Draconis is neary 50 times that of the Sun...
, ζ Draconis
Zeta Draconis
Zeta Draconis is a star in the constellation Draco. It shares the Arabic name Al dhi'bah with several other stars in the constellation, and, together with Eta Draconis, it also shares the name Al dhibain...
, υ Draconis, 73 Draconis and 23 Cassiopeiae. Consequently, γ Cephei itself is known as , representing , meaning The Second Imperial Guard or Minor Guard
Planetary system
A planet orbiting Gamma Cephei A was tentatively identified by a Canadian team consisting of Bruce Campbell, Gordon Walker and Stephenson Yang in 1988. Its existence was also announced by Anthony Lawton and P Wright in 1989. This would have been the first confirmed extrasolar planet and its ostensible discovery was based on the same radial velocityRadial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
technique later used successfully by others. However, the claim was retracted in 1992 because the quality of the data was not good enough to establish discovery. But in 2002, evidence of the planet was considerably strengthened by new measurements by Artie Hatzes and his collaborators at the McDonald Observatory
McDonald Observatory
The McDonald Observatory is an astronomical observatory located near the unincorporated community of Fort Davis in Jeff Davis County, Texas, United States. The facility is located on Mount Fowlkes and Mount Locke in the Davis Mountains of West Texas...
.
The secondary star B orbits A at only 9.5 times the semimajor axis of A's planet. This is the smallest ratio of planetary orbits in multiple systems. The orbits are probably coplanar.
External links
- http://jumk.de/astronomie/exoplanets/errai.shtml
- McDonald Observatory: Planet Search finds first planet orbiting close-in binary star
- A Planetary Companion to the Binary Star Gamma Cephei
- SolStation: Errai 2
- Extrasolar Visions: Gamma Cephei A + Gamma Cephei Ab + Gamma cephei B